The worst blackout in more than a decade hit Athens and southern Greece on Monday, leaving millions sweltering in a heat wave and raising concerns about whether the lights will go out at next month's Olympics.

The government blamed the outage on "mismanagement" of the electricity grid. Still, officials promised the network was ready to handle the Aug. 13-29 Olympics.

But it was yet another hurdle in Athens' attempt to convince the world it is ready to host well-run and safe games. Olympics preparations have come under criticism because of construction delays and concerns over security arrangements to stop terror attacks.

The blackout knocked out air conditioners as afternoon temperatures soared to 104 degrees Monday. The power failure created enormous traffic jams from failed traffic signals and stalled electric trolleys. Hundreds of passengers on the Athens subway were forced to leave trains and walk, and the fire department received hundreds of calls about people trapped in elevators.

In one embarrassing moment for the government, Transport Minister Mihalis Liapis was making a test run to showcase a new Olympic rail link from central Athens to the airport — and got stranded en route when the power failed.

Government officials said generators had to be pressed into service at Olympic venues.

The domino-effect outages were traced to an imbalanced flow of electricity that shut down four power-generating stations, according to a senior government official who spoke on condition of anonymity.

Greece's Public Power Corp. did not explain what caused the blackout, saying only that it knocked out four major plants. The company — an Olympic sponsor — blamed the state-owned grid operator for the outage.

Others members of the Greek government flooded the media with pledges that the Olympics are in no danger of going dark.

There was more than enough power for a "smooth and uninterrupted" flow of electricity to the whole country during the Olympics, Development Minister Dimitris Sioufas said.

He also noted that five additional electrical substations plan to go into operation next month to lower the chances of power spikes. Substations are electrical facilities where the voltage of incoming circuits is controlled and the current is distributed to other lines.

The Athens Olympic Organizing Committee, meanwhile, gave assurances that generators would allow the games to proceed even if there's another big outage. It said protections are in place for everything crucial to the games, including timers and broadcasting equipment.

"A similar incident would not affect the competition schedule and the broadcasting of the games," the committee said of the outage.

Athens is home to nearly 5 million people, and an estimated 2 million more are expected in Greece this August.

The blackout began at 12:39 p.m. in Athens and quickly spread.

Outages were reported as far as Larissa, 155 miles north of Athens, and the port of Kalamata, 175 miles to the south. It also included some islands in the Aegean and Ionian seas.

Power was restored to 70 percent of the region in about an hour and to all of Athens in just over three hours. Remote areas were affected longer.

It was initially believed the power outage was due to increased demand in air conditioner usage because of the heat, the chief cause for most blackouts in Greece. Last summer, Athens was left without power for several hours because of increased air conditioner use.

However, Sioufas said, "the cut was not due a lack of power, but mainly due to mismanagement of the high-voltage grid."

July is traditionally the hottest month in Greece, with highs often hitting 104. The Greek weather service said average highs in Athens next month should be about 90.

The power company has been maintaining high water reserves for its hydroelectric plants and plans to buy extra electricity from neighboring Bulgaria if needed during the games.

It also has a plan to redistribute power from the provinces to Athens — an action that was taken Monday.